1. Field of the Invention
The present invention relates to a carrier for use in a two-component developer developing an electrostatic image, to a method for preparing the carrier, and a two-component developer using the carrier and a toner. In addition, the present invention also relates to a developer container, and an image forming method and a process cartridge using the two-component developer.
2. Description of the Related Art
Electrophotographic image forming methods typically include the following processes:
(1) Forming an electrostatic latent image on an image bearing member such as a photoreceptor;
(2) Developing the electrostatic latent image with a developer including a toner to form a toner image on the image bearing member;
(3) Transferring the toner image onto a recording material;
(4) Fixing the toner image on the recording material, resulting in formation of an output image.
Recently, in the field of electrophotography, transition is rapidly being made from monochrome imaging to full-color imaging, and the market for full-color imaging is rapidly expanding.
In full-color imaging, all the color images are formed by forming primary color toner images using four color toners, i.e., yellow, magenta, cyan and black toners, while forming secondary color toner images by overlaying two or more of the primary color toner images. Therefore, in order to prepare a full-color image having a good combination of color reproducibility and clearness, the fixed color toner image preferably has a smooth surface to reduce light scattering at the surface. For this reason, color images produced by conventional full-color image forming apparatus typically have a relatively high glossiness of from 10% to 50%.
With respect to the image fixing method, contact heat fixing methods in which a heated fixing member such as a heat roller or a belt is contacted with a toner image upon application of pressure thereto are widely used. Such contact heat fixing methods have advantages of fixing a toner image at a high speed and a high heat efficiency while imparting a good combination of glossiness and transparency to the toner image. However, the contact heat fixing methods have a drawback in that they often cause an offset problem, in which apart of a toner image is adhered to a fixing member, and the adhered toner is transferred again to the image or another image, resulting in formation of an abnormal image, because the toner image is contacted with the fixing member upon application of heat and pressure to be melted.
In attempting to prevent occurrence of the offset problem, typically fixing methods are used in which a fixing roller having a surface made of a material having good releasability such as silicone rubbers and fluorine-containing resins is used while applying a toner adhesion preventing agent such as silicone oils to the surface of the fixing roller. Although the fixing methods are effective in preventing occurrence of the offset problem, the methods have a drawback in that, since an oil applicator has to be provided, the fixing device becomes unacceptably large. Therefore, recent monochrome image forming apparatuses tend to use toner having a relatively high melt viscoelasticity and including a release agent in combination with an oil-less fixing device or an oil micro-coating fixing device, in which a small amount of oil is applied to a fixing member.
Similarly, oil-less fixing methods are often used for full-color image forming apparatuses to miniaturize the fixing devices thereof and simplify the configuration. However, since full-color image forming apparatuses preferably produce glossy images as mentioned above, color toners used therefor preferably have a lower viscoelasticity than toners used for monochrome image forming apparatuses, thereby increasing the chance of occurrence of the offset problem. Therefore, it is difficult for full-color image forming apparatuses to use an oil-less fixing device.
In addition, toner including a release agent has drawbacks in that transferability of the toner to a recording material deteriorates because of having high adhesiveness to the surface of carrier, and a toner filming problem in that a film of toner is formed on the surface of the carrier used in combination of the toner, resulting in deterioration of the charging ability and durability (life) of the carrier
On the other hand, coated carriers in which a resin having a low surface energy such as fluorine-containing resins and silicone resins is uniformly applied on a core material thereof are provided in order to prolong the life thereof, i.e., to prevent occurrence of the toner filming problem and other problems such that the surface of the carriers is oxidized, the moisture sensitivity of the carriers deteriorates, the carriers are adhered to image bearing members, and the carriers damage and abrade the surface of image bearing members, and to control the polarity and quantity of charge of the carriers.
Specific examples of the coated carriers having a surface coated with a resin having a low surface energy include a carrier having a cover layer formed by using a room temperature crosslinking silicone resin and a positively chargeable nitrogen-containing resin; a carrier having a cover layer formed of a material including at least a modified silicone resin; a carrier having a cover layer formed by using a room temperature crosslinking silicone resin and a styrene-acrylic resin; carriers having multiple cover layers formed by using silicone resins, wherein the cover layers may have poor adhesiveness with each other; a carrier having a cover layer including a silicone resin and silicon carbide; a positively chargeable carrier having a cover layer formed of a material having a critical surface tension of not greater than 20 dyne/cm; and a developer consisting of a carrier having a cover layer formed by using a coating agent including a fluorinated alkylacrylate, and a toner including chromium-containing azo dye.
Moreover, in attempting to prevent occurrence of a spent toner problem, in which spent toner is adhered to the surface of a carrier, resulting in deterioration of the charging ability of the carrier, there are proposals for carriers having a cover layer including a particulate inorganic material such as silica so that the spent toner adhered to the carrier is abraded by the particulate inorganic material. However, since the silica used for the cover layer are spherical, the abrasion effect is slight, and in addition the silica included in the cover layer is easily released therefrom under heavy stress applied to the carrier, for example, in a thin developer layer developing method in which a thin developer layer is formed on a developer bearing member to develop an electrostatic latent image. Therefore, the carrier cannot maintain the abrasion effect for a long period of time, thereby degrading the charging ability of the carrier and decreasing the charge quantity of the toner, resulting in occurrence of a toner scattering problem in that toner scatters around a developing device, thereby contaminating parts of the developing device and the image forming apparatus, and a background development problem in that the background of an image is soiled with toner particles having an insufficient charge quantity.
Further, in order to produce high quality images, the diameter of the particles that constitute the toner is being specification reduced. When images are formed at a high-speed using such small toner particles, the spent toner problem is easily caused. In this regard, when a wax is included in the toner as a release agent, the amount of spent toner adhered to the carrier seriously increases, thereby degrading the charging ability of the carrier and decreasing the charge quantity of the toner, resulting in occurrence of the toner scattering problem and the background development problem.
In full-color image formation systems, when spent toner is adhered to the surface of a carrier, or the cover layer of a carrier is abraded or released, the resistance of the carrier and the amount of toner born by the surface of the carrier change, resulting in a change of image density (particularly image density of highlighted portions). In addition, when a filler included in a cover layer of a carrier is released therefrom due to abrasion of the cover layer and mixed with a color toner used in combination therewith, the color of the color toner (particularly yellow toner) is changed, resulting in deterioration of the color reproducibility of images.
For these reasons, the inventors recognized that there is a need for a carrier which can produce high quality images in combination with toner without causing the above-mentioned problems such as the spent toner problem, the toner scattering problem, and the background development problem.